The present invention relates to an apparatus and a method for collecting and redistributing a flow of a descending liquid to a structured packing in an exchange column for heat and/or mass transfer processes. The apparatus and method have particular application in cryogenic air separation processes utilizing distillation, although the apparatus and method may be used in other heat and/or mass transfer processes. The present invention also relates to methods for assembling an apparatus for collecting and redistributing a flow of a descending liquid to a structured packing in an exchange column used in such processes.
The term “column”, as used herein, means a distillation or fractionation column or zone, i.e., a column or zone wherein liquid and vapor phases are countercurrently contacted to effect separation of a fluid mixture, such as by contacting of the vapor and liquid phases on packing elements or on a series of vertically-spaced trays or plates mounted within the column.
The term “packing” means solid or hollow bodies of predetermined size, shape, and configuration used as column internals to provide surface area for the liquid to allow mass transfer at the liquid-vapor interface during countercurrent flow of two phases. Two broad classes of packings are “random” and “structured”.
“Random packing” means packing wherein individual members do not have any particular orientation relative to each other or to the column axis. Random packings are small, hollow structures with large surface area per unit volume that are loaded at random into a column.
“Structured packing” means packing wherein individual members have specific orientation relative to each other and to the column axis. Structured packings usually are made of expanded metal or woven wire screen stacked in layers or as spiral windings; however, other materials of construction, such as plain sheet metal, may be used.
The terms “hole” and “aperture” are used interchangeably herein to mean an opening through which a fluid may pass. Although circular holes are shown in the drawings, the holes may have other shapes, including irregular as well as regular shapes.
Cryogenic separation of air is carried out by passing liquid and vapor in countercurrent contact through a distillation column. A vapor phase of the mixture ascends with an ever increasing concentration of the more volatile components (e.g., nitrogen) while a liquid phase of the mixture descends with an ever increasing concentration of the less volatile components (e.g., oxygen).
Various packings or trays may be used to bring the liquid and gaseous phases of the mixture into contact to accomplish mass transfer between the phases. The use of packing for distillation is standard practice and has many advantages where pressure drop is important. However, packed column performance is very dependent on creating and maintaining a balance between the downward flow of liquid and the upward flow of vapor locally in the packing. The distribution of the liquid and the vapor within the packing is influenced by the initial presentation of these fluids to the packing.
Initial presentation of liquid and vapor to the packing is usually made by means of distributors. A liquid distributor, the role of which is to irrigate the packing uniformly with liquid, is located above the packing, while a vapor distributor, the role of which is to create uniform vapor flow below the packing, is located below the packing.
Maldistribution of liquid in packed two-phase mass/heat transfer columns has long been recognized as leading to poor mass/heat transfer efficiency. Liquid maldistribution may be due to: initially presenting the liquid to the packing non-uniformly; maldistributed vapor flow which forces liquid maldistribution through the mechanism of vapor/liquid shear; and the packing itself, which has an inherent “natural distribution” to which the liquid distribution tends, regardless of initial liquid presentation. It is well known that liquid tends to accumulate near the walls of columns containing structured packing as shown in U.S. Pat. No. 6,286,818 B1 (Buhlman), U.S. Pat. No. 6,513,795 B2 (Sunder), and numerous publications in the published literature (e.g., Stoter et al., “Measurement and modeling of liquid distribution in structured packings,” Chem. Eng. J. (1993) 53 55).
Various approaches in the prior art address liquid accumulation near or on the wall of the column. The approaches fall into three categories: A) those that attempt to redirect the liquid which is on or near the wall back into the packing locally; B) those that attempt to re-collect all of the liquid flowing in the packing and on/near the wall, mix it to some degree, and redistribute it; and C) other approaches. The three categories are discussed below.
Category A: Re-introduction of Liquid Locally
The theory behind this common approach is that the liquid flowing on the wall or along the outer periphery of the structured packing can be re-introduced at the periphery of the packing and will naturally flow back into the bulk of the packed bed. Most structured packings used in this approach include one or more short bands of “wall-wiper” material (typically a metal foil or gauze) which is folded against the inside diameter of the column and against the periphery of the structured packing to reduce “liquid and vapor bypass.”
U.S. Pat. No. 5,224,351 (Jeannot, et al.) discloses several types of wall-wipers that collect liquid from the wall and wall region of the packing and attempt to re-introduce the liquid within the same packing layer or at the periphery of the layer below. These approaches have a claimed advantage of reducing vapor by-pass, as disclosed in EP997189 A1 (Klotz, et al.) However, as taught in U.S. Pat. No. 6,286,818 B1 (Buhmann), these conventional designs still result in accumulation of liquid on the wall or near-wall region of the packing. Furthermore, the “wall-wiper” must be applied to each layer, or very frequently along the height of the packed bed. Applicants believe that the inefficiency of this approach lies in the fact that the liquid re-introduced to the packing from the wall or near-wall region enters the packing at the outer periphery, and will just as likely flow back toward the wall/near-wall region as flow toward the center of the column.
Modifications of the edge of the packing also have been proposed as a means for redirecting the liquid back into the packing. Specific geometries for edge region folding and cutting and tools for producing these features have been disclosed in U.S. Pat. No. 5,224,351 (Jeannot, et al.). This approach is penalized by the increased cost for tooling and special handling of each sheet of structured packing, as well as the increased assembly costs associated with the individual segments (or “bricks”) of structured packing which must necessarily be assembled with the specific orientation mandated by the edge modification. Furthermore, as in the “wall-wiper” approach, the packing-edge-modification approach relies on the redirected liquid to tend to flow toward the center of the column cross-section after re-direction, whereas the liquid actually may flow either to or away from the wall and tend to flow toward the wall in net.
Finally, baffles which extend a small distance into the packing from the inside diameter of the column may be added to redirect liquid flow, as disclosed in EP0684060 B1. This approach suffers from the same shortcomings as the previously described approaches, which tend to re-introduce liquid close to the wall region where the liquid may re-accumulate rapidly.
Category B: Liquid Collectors/redistributors
The approaches in this category require a liquid collection/mixing/re-distribution device to handle the entire liquid flow in the column while attempting to minimize perturbations on the distribution of ascending vapor in the column. The devices tend to be large and include many massive parts, and thus incur large manufacturing costs and the added costs associated with additional column height.
U.S. Pat. No. 5,240,652 (Taylor, et al.) is an example of a “standard” approach to liquid distribution in packed columns. Liquid is captured from the entire cross-section of the packing in a “pre-distributor,” which then mixes the entire liquid flow in a trough or pot before distributing the liquid somewhat crudely to a secondary distributor, which then distributes the entire liquid flow to the packing below with a high degree of uniformity. Vapor risers penetrate both the pre-distributor and the secondary distributor. The cost associated with the column height required to get uniform liquid flow across the column cross-section from the secondary distributor and to accommodate the pre-distributor is very significant. Furthermore, manufacturing and installation costs for this type of design also are high. Because of the high costs, these distributors are only used at the top of a packed section in which liquid streams are introduced or withdrawn, or to interrupt a very tall packed section so as to effect complete mixing and re-distribution at an intermediate point in the tall packed section.
EP0782877 B1 (Billingham, et al.) is a variant of the standard liquid distributor design (U.S. Pat. No. 5,240,652) in which a baffle is suspended above the distributor to effect enhanced liquid mixing.
EP0684060 B1 is a variant of the standard liquid distributor design, in which the irrigation hole density is varied across the cross-section of the column to reduce the resulting maldistribution created by the wall/near-wall region of the packing.
U.S. Pat. No. 5,464,573 (Tokerud, et al.) is an alternative to the standard liquid distributor design and is used to save column height/cost. Use of a pre-distributor is avoided by capturing liquid directly from the packing located above by vanes on every trough. In one embodiment, an annular ring is used to capture liquid flowing on the wall. The ring is fitted with features (e.g., lateral troughs) to direct the liquid into the bulk of the liquid distributor.
EP0879626 A2 (Hine, et al.) is an alternative to the standard liquid distributor design and is used to save column height/cost. Use of a pre-distributor is avoided by partially mixing the captured liquid within a series of small troughs on top of the vapor risers.
U.S. Pat. No. 5,132,055 (Alleaume, et al.) discloses a liquid distributor which also acts as a support for the packed section above. The distributor consists of a heavy plate which extends almost entirely across the column cross-section. The plate is fitted with sturdy, inverted troughs for vapor flow and packing support. Holes in the bottom of the plate distribute the liquid to the packing below. The distributor is sealed to the column periphery such that liquid flowing on the wall of the column is captured with the rest of the liquid descending from the packing. There is no pre-distributor, and no special means for mixing liquid captured from the wall with the rest of the captured liquid.
U.S. Pat. No. 5,224,351 (Jeannot, et al.) discloses a means to capture liquid flowing on the wall or packing periphery of a packed section located above a liquid distributor as described in U.S. Pat. No. 5,132,055 (Alleaume, et al.), with special provision to bring the captured liquid into the center of the distributor to effect better mixing with the bulk liquid collected in the distributor.
Category C: Other Approaches
These approaches to redistribution require the use of a different type or density of packing between two larger sections of predominant packing. The main drawback of these approaches is that they do not specifically address the maldistribution associated with the column wall/packing periphery.
U.S. Pat. No. 6,286,818 B1 (Buhlmann) discloses a means to redistribute liquid in a packed section by using a short section of packing which is dissimilar from the rest of the packed section. The packing used in the short section is intended as a means of redistribution of liquid. U.S. Pat. No. 6,513,795 B2 (Sunder) discloses a means to correct liquid maldistribution by using “mixed resistance structured packing” consisting of structured packing with low resistance to flow in the central core of the column and structured packing with high resistance to flow in the outer annulus surrounding the central core. Both of these approaches attempt to address liquid maldistribution by changing the characteristics of the structured packing within the column, but no special provisions are made to address the predominance of liquid on the wall of the column or in the near-wall region of the packing.
WO 0166213 A1 also proposes the use of packing as a distribution means by making use of hydraulic flooding. Hydraulic flooding occurs when the hydrodynamic shear of the upwardly flowing vapor on the downwardly flowing liquid balances the tendency of the liquid to flow downward because of gravity. What results is a locally high density of liquid approaching the density of the liquid froth found on standard perforated distillation trays. This reference teaches use of a short section of high density packing between two taller sections of lower density packing to create local flooding conditions in the high density packing and thereby redistribute the liquid more uniformly in the lower packed section. No special provisions are made for capturing liquid flowing on or near the wall of the column. A disadvantage of this approach is that it introduces additional pressure drop into the vapor phase as it passes through the high density (flooded) packing.
The categories of the three approaches are summarized in the Table below, which includes the associated disadvantages/shortcomings of each approach.
CategoryDescriptionDisadvantage/ShortcomingA. Re-introductionDevices or packingThe liquid is re-introducedof liquid locallyfeatures located at theclose to the wall of thepacking periphery actcolumn at the packing peri-to re-introduce liquidphery where it is likely toto the packingreturn to the wall regionvery quickly after redistri-bution.B. Liquid collectors/These are devicesBecause these devicesredistributorswhich capture all ofhandle the full liquid flow,the liquid flowingthey are necessarily big,from a packed section,complicated, and thereforeand, to varyingcostly.degrees, attempt tomix it and redistributeit.C. Other approachesDifferentThis approach does nottypes/densities ofexplicitly handle thepacking have beenmaldistribution associatedproposed aswith the wall flows, andredistributors withinthus does not address thesections ofmajor mechanism contri-predominantly otherbuting to liquid maldistri-packingsbution in packings.
It is desired to have an apparatus and a method for collecting and redistributing a flow of a liquid descending in an exchange column which captures the liquid on the wall of the column or the region of the structured packing in the column to redistribute the liquid far into the interior of the structured packing away from the column wall.
It is further desired to have an apparatus and a method for collecting and redistributing a flow of a liquid descending in an exchange column which requires relatively little additional column height and associated costs.
It is still further desired to have an apparatus and a method for collecting and redistributing a flow of a liquid descending in an exchange column which does not cause a significant added pressure drop in the vapor ascending in the column.
It is still further desired to have an apparatus and method for collecting and redistributing a flow of a liquid descending in an exchange column which can be integrated with conventional holddown grate/support grate designs.
It is still further desired to have an apparatus and a method for collecting and redistributing a flow of liquid descending in an exchange column which decrease the likelihood of malperformance caused by gross liquid maldistribution in a packed column by mitigating the effects of liquid maldistribution.
It is still further desired to have a new, more efficient apparatus and method for collecting and redistributing a flow of a liquid descending in an exchange column.
It is still further desired to have an apparatus and a method for collecting and redistributing a flow of a liquid descending in an exchange column which overcome the difficulties, problems, limitations, disadvantages, and deficiencies of the prior art to provide better and more advantageous results.
It is still further desired to have a method for assembling an apparatus for collecting and redistributing a flow of a liquid descending in an exchange column which affords better liquid distribution than the prior art, and which also overcomes many of the difficulties and disadvantages of the prior art to provide better and more advantageous results.
It is still further desired to have an apparatus and a method for collecting and redistributing a flow of a liquid descending in an exchange column which show high performance characteristics for cryogenic applications, such as those used in air separation, and for other heat and/or mass transfer applications.
It also is desired to have a more efficient air separation process utilizing an apparatus for collecting and redistributing liquid which is more efficient than the prior art.